Updated: Wednesday, October 23rd, 2013 at 5:42pm

Bicyclists become invisible and their dim lights become streaks of color in the brightness of moonlight reflected off the snow-white gypsum sand at White Sands National Monument. (Dean Hanson / Journal)

Twice a year, cyclists have a brief opportunity to explore the Alice in Wonderland world of White Sands National Monument under moonlight conditions that render the vast gypsum desert even more surreal than it appears during the day.

April and October are the months when the air temperature, park opening hours and Earth’s closest neighbor align to allow a bike ride through the dunes under a full moon.

On those occasions, riders can cruise along the 16-mile Dunes Road for a three-hour period after dark. The route takes them deep into the heart of the shifting sands where the moonlit dunes rise on each side of the roadway like billowy mounds of snow.

White Sands National Monument, operated by the U.S. National Park Service, is located in the 275-squaremile gypsum sand desert in the Tularosa basin between the San Andres Mountains to the west and the Sacramento Mountains to the east. The rides begin at the visitors entrance, which is off U.S. 70 about 15 miles southwest of Alamogordo.

Park staffers said the event typically draws riders from throughout New Mexico, other states and even a few from Europe.

I participated in the April 16 Full Moon ride along with nearly 200 other cyclists of all ages and abilities. White Sands staff and volunteers from the El Paso Bicycle Club help riders check in, making sure they have helmets and lights. They tape a glowing light stick to each bike for added visibility.

Riders are allowed to start about an hour after the park closes, once all the tourists in cars have left. In April the start is at 9 p.m.; in October the ride begins an hour earlier. Cyclists have to pay a $5 entry fee for the event. Some discounts are available.

The route initially follows a paved road. After a few miles it becomes covered with sand, which can pose a problem for cyclists with road bikes that have skinny tires. Cruiser bikes or mountain bikes with chunky tires are definitely preferable for this event.

Although the sandy surface is rutted like a washboard in places, the route is level, making it an easy ride for even for the casual weekend cyclist.

Once among the dunes, the route follows a loop, passing a picnic area where several people I rode with chose to stop and take in the view or take pictures.

“It was surreal and very sci-fi-ish,” said Mary Ann Sweeney, a member of the New Mexico Touring Society bicycle club, who traveled from Albuquerque for the event.

Under the ghostly light, my eyes too played tricks; yucca plants looked like human figures standing in the sand. Elsewhere, the road and dunes appeared to merge. Riders were visible only by the pin pricks of red and white on their bikes and the greenish florescence of their light sticks.

Afterward, Ron Taylor, a rider from Albuquerque, was jubilant about the ride.

“Wasn’t that a magnificent biking experience!” Taylor said.

White Sands limits participation in the full moon ride to 200 cyclists. Registration is required, and online registration forms are posted one month before the event. Places typically fill up within hours, and some would-be riders are disappointed.

“We’ve been trying to get in for the past four years and they’ve always been filled up by the time we got signed up,” said Shirley Kennedy, a rider from Rio Rancho who participated for the first time this month.

The park has been holding the full moon rides for the past 12 years. October events are more popular with families because the shorter days allow the rider to start earlier, said Becky Wiles, the chief of interpretation at White Sands.

She said April and October are the only two months when the weather and the park closing time make it feasible to hold the ride. In winter it is too cold, and in the summer the park stays open until 10 p.m., so riders could not be allowed in until 11 p.m., Wiles said.

Several times in recent years, the event nearly has been canceled because of rain or high winds.

“This past ride was some of the best conditions I’ve seen for a full moon bike ride,” she said.